In video cameras, a so-called TV-AF method is recently a standard method for focus control, in which an AF evaluation value signal is generated based on sharpness (a contrast state) of images detected from video signals obtained by photoelectrical conversion of an object image with an image-pickup element, and then focus control is performed so that a position of a focus lens, where the AF evaluation value signal may become maximum, is searched.
Generally, a high frequency component extracted from the video signals is used as the AF evaluation value signal in the TV-AF method. The AF evaluation value signal becomes larger as the position of the focus lens approaches an in-focus point (in-focus position) when taking an image of a normal object, as shown in FIG. 2. The position where the level (value) of the AF evaluation value signal reaches the maximum is the in-focus point.
Moreover, there is an internal phase difference detecting method as another AF method, in which a luminous flux that have passed through a pupil of an image-taking optical system is divided into two luminous fluxes, and a pair of focus detecting sensors (line sensors) receive the two luminous fluxes, respectively. An out-of-focus amount and an out-of-focus direction can be directly calculated based on a displacement amount between signals output according to the light-receiving amounts of the sensors, that is, relative positional displacement amount of the luminous fluxes in the dividing direction thereof.
By the use of this method, the out-of-focus amount and the out-of-focus direction can be obtained if an accumulation operation in the focus detecting sensor is performed once. Therefore, a high-speed focus adjustment operation becomes possible.
Moreover, there is an external distance-measuring method using a distance-measuring sensor that is independent of the image-taking lens. In this method, a luminous flux from an object is divided into two luminous fluxes, and two focus detecting sensors receive the two luminous fluxes respectively. A displacement amount between signals output according to the light-receiving amounts of the sensors, that is, a relative positional displacement amount of the luminous fluxes in the dividing direction thereof, is detected, and an object distance is calculated by triangulation.
There are other methods using an external distance-measuring sensor such as a method in which an object distance is measured based on a transmission speed of ultrasonic waves detected by an ultrasonic sensor, and a method in which an object distance is measured by triangulation using an infrared sensor.
Furthermore, an AF method as a combination of the TV-AF method and an AF method other than the TV-AF method has been proposed, in which, for instance, a focus lens is moved to the vicinity of an in-focus point by the internal phase difference detecting method, and then moved to the in-focus point more accurately by the TV-AF method (see Japanese Laid-Open Patent Application No. H05-64056).
However, the TV-AF method includes the following disadvantages. First, since the AF evaluation value signal is generated from video signals, the AF evaluation value signal may fluctuate according to the changes of objects and image-taking conditions. In this case, the focus lens may be driven in a false direction (in a direction away from the in-focus point) because of the misjudgment of an in-focus direction.
Moreover, in a case where, from a state in which the image-taking optical system has focused on a certain distance object, the object moved out of an image-taking area, if the AF evaluation value for an out-of-focus object that does not exist at the certain distance is equal to the AF evaluation value for the certain distance object, the AF operation may not be restarted, and therefore out-of-focus images may be taken continuously.
In the combination of the TV-AF method and an AF method other than the TV-AF method, the focus lens is moved to the vicinity of the in-focus point by an AF method other than the TV-AF method, and the AF method is then changed to the TV-AF method only when the AF evaluation value is larger than a predetermined level (when the focus lens is within an in-focus range) to move the focus lens to the in-focus point.
However, in a case where the AF evaluation value has changed by the change of the object's contrast despite in a state in which the object distance has not changed, the focus state may change because of the operation of the TV-AF in the image-taking condition requiring no movements of the focus position originally, and unnecessary image blur may be generated.
Such an unnecessary TV-AF operation is allowable in an image-taking apparatus in which image recording is not performed during the AF operation. However, in an image-taking apparatus in which video image recording is also performed during the AF operation, image blur caused by the unnecessary TV-AF operation will be recorded.
Furthermore, if the AF method with the external distance-measuring sensor, which is an AF method other than the TV-AF method, is adopted, parallax between the image-taking area of the image-taking lens and the distance-measuring area of the distance-measuring sensor may be generated. In this case, the focus lens may move according to the change of the object distance detected by the external distance-measuring sensor though it is in an in-focus state, and image blur may occur.